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Bone screw displacement under lateral loading
799
Abstracts SESSION B
REDUCTION
OF AXIAL PLATE SCREW FORCE IN A CORTICAL
BONE
STATIC AND DYNAMIC
LOADING MODE H. BR~~MMER and
P.
(University of Ulm, Department Experimental Surgery, Ulm, Germany)
HUTZSCHENREUTER
of Surgery,
In vitro, osteotomies of sheep tibia, gap-condition, were plated by ASIF-method (6round hole dog plates, 3.5 mm cortical screws). For registration of axial screw forces strain gage washers were used. Sinusoidal tension-compression loading in axial direction of the bone were generated with a hydropuls equipment. We distinguish static mode, from the insertion moment of the screw up to 72 hr, simulating the patient’s lying in bed, postoperatively, without any load on the bone. And dynamic load, following up static mode, simulating patient’s monthly weight bearing, with critical frequencies on man and 250.000 cycles totai. Results : we distinguish two groups, such whose screws are tightened only one time and such with three times tightening. Static mode: one time group had 15 per cent more reduction than three times group. Dynamic mode: both groups had a reduction of 6 per cent. Static mode following up dynamic mode: both groups had less reduction than before dynamic mode. BONE SCREW DISPLACEMENT UNDER LATERAL LOADING
D.
J.
MARTIN,
CORDEY,
B.
A. RAHN and S. M. PERREN(Laboratory
for Experimental
Surgery, Davos, Switzerland) Bone screws were originally designed to withstand tension load along the screw axis. Compression plates compress the fracture site by applying a lateral bending load to the screw head. To learn the mechanical response of the screw to this bending load, A0 4.5 mm cortical screws mounted in bone were bent in uirro. Results show that with the screw head center 3.8 mm above the bone, 940 N (S.E. = 3.5%) is required to begin plastic screw bending, and 1350 N is needed for 0.25 MM plastic bend. These forces displace the screw head 0.3 mm and 0.7 mm, respectively. Histology showed that bone compression occurred near the bone surface, lowering the bending fulcrum and lengthening the lever arm on the screw. These results suggest loading the screw into its plastic range to achieve optimal compression, support the use of spherical screw heads and low screw seating tn the plate, and influence current thinking regarding compression plate mechanics. THE PREDICTION
J.
CORDEY
OF POSTOPERATIVE
COMPRESSION
IN CORTICAL
and S. M. PERREN(Laboratory for Experimental Institute, Davos, Switzerland)
BONE SCREWS
Surgery, Swiss Research
Screws undergo fracture fixation through their axial force. We found previously in Lliro a reduction of axial force to be purely mechanical during the first 3-4 weeks. The laws of reduction remained to be investigated. Corttcal screws were tightened on human cadaveric tibiae. The axial force was measured using a piezoelectric washer. Different amounts of axial force were applied for a duration of 10 min to several hours. The axial force decreases quasi-linearly with the logarithm of elasped time. The slope of force reduction is proportional to the initial force. Using the data of the first 10 min, we could predict well the behaviour of axtal force during the first three hours, and fairly predict the behaviour over 3-4 weeks. Simple viscoelastic models cannot describe the axial force reduction. Both viscoelasticity of bone and viscous friction between thread and bone are to be taken in account. RELATIONSHIP
BETWEEN
PLATE-BONE
CONTACF
AREA AND BLOOD SUPPLY
IN INTERYAL
FIXATION
U. LUETHI,T. DUELANDand B. A. RAHN (Laboratory for Experimental Research Institute, Davos, Switzerland)
Surgery, Swiss
Remodelling with temporary porosis has been attributed to stress protection. However Gunst et a[. related It to altered circulation. Thecontact area of plates were determined and their influences on cortical bone perfusion. Plates, adapted to human cadaver bones by experienced surgeons, were 13-60 per cent (mean 329,) in contact with the bone. No correlation was found between the applied force and the achieved contact area. Normally the contact pressure does not approach the ultimate compressive strength of bone. In sheep bones disulphine blue vital staining was generally absent over the whole area benearh the piare. Haversian canals with blood flow (blue) or without flow (red) could be used to evaluate the blood suppI!
Abstracts SESSION B
REDUCTION
OF AXIAL PLATE SCREW FORCE IN A CORTICAL
BONE
STATIC AND DYNAMIC
LOADING MODE H. BR~~MMER and
P.
(University of Ulm, Department Experimental Surgery, Ulm, Germany)
HUTZSCHENREUTER
of Surgery,
In vitro, osteotomies of sheep tibia, gap-condition, were plated by ASIF-method (6round hole dog plates, 3.5 mm cortical screws). For registration of axial screw forces strain gage washers were used. Sinusoidal tension-compression loading in axial direction of the bone were generated with a hydropuls equipment. We distinguish static mode, from the insertion moment of the screw up to 72 hr, simulating the patient’s lying in bed, postoperatively, without any load on the bone. And dynamic load, following up static mode, simulating patient’s monthly weight bearing, with critical frequencies on man and 250.000 cycles totai. Results : we distinguish two groups, such whose screws are tightened only one time and such with three times tightening. Static mode: one time group had 15 per cent more reduction than three times group. Dynamic mode: both groups had a reduction of 6 per cent. Static mode following up dynamic mode: both groups had less reduction than before dynamic mode. BONE SCREW DISPLACEMENT UNDER LATERAL LOADING
D.
J.
MARTIN,
CORDEY,
B.
A. RAHN and S. M. PERREN(Laboratory
for Experimental
Surgery, Davos, Switzerland) Bone screws were originally designed to withstand tension load along the screw axis. Compression plates compress the fracture site by applying a lateral bending load to the screw head. To learn the mechanical response of the screw to this bending load, A0 4.5 mm cortical screws mounted in bone were bent in uirro. Results show that with the screw head center 3.8 mm above the bone, 940 N (S.E. = 3.5%) is required to begin plastic screw bending, and 1350 N is needed for 0.25 MM plastic bend. These forces displace the screw head 0.3 mm and 0.7 mm, respectively. Histology showed that bone compression occurred near the bone surface, lowering the bending fulcrum and lengthening the lever arm on the screw. These results suggest loading the screw into its plastic range to achieve optimal compression, support the use of spherical screw heads and low screw seating tn the plate, and influence current thinking regarding compression plate mechanics. THE PREDICTION
J.
CORDEY
OF POSTOPERATIVE
COMPRESSION
IN CORTICAL
and S. M. PERREN(Laboratory for Experimental Institute, Davos, Switzerland)
BONE SCREWS
Surgery, Swiss Research
Screws undergo fracture fixation through their axial force. We found previously in Lliro a reduction of axial force to be purely mechanical during the first 3-4 weeks. The laws of reduction remained to be investigated. Corttcal screws were tightened on human cadaveric tibiae. The axial force was measured using a piezoelectric washer. Different amounts of axial force were applied for a duration of 10 min to several hours. The axial force decreases quasi-linearly with the logarithm of elasped time. The slope of force reduction is proportional to the initial force. Using the data of the first 10 min, we could predict well the behaviour of axtal force during the first three hours, and fairly predict the behaviour over 3-4 weeks. Simple viscoelastic models cannot describe the axial force reduction. Both viscoelasticity of bone and viscous friction between thread and bone are to be taken in account. RELATIONSHIP
BETWEEN
PLATE-BONE
CONTACF
AREA AND BLOOD SUPPLY
IN INTERYAL
FIXATION
U. LUETHI,T. DUELANDand B. A. RAHN (Laboratory for Experimental Research Institute, Davos, Switzerland)
Surgery, Swiss
Remodelling with temporary porosis has been attributed to stress protection. However Gunst et a[. related It to altered circulation. Thecontact area of plates were determined and their influences on cortical bone perfusion. Plates, adapted to human cadaver bones by experienced surgeons, were 13-60 per cent (mean 329,) in contact with the bone. No correlation was found between the applied force and the achieved contact area. Normally the contact pressure does not approach the ultimate compressive strength of bone. In sheep bones disulphine blue vital staining was generally absent over the whole area benearh the piare. Haversian canals with blood flow (blue) or without flow (red) could be used to evaluate the blood suppI!